To calculate the position of a device within a wireless cellular network (e.g., a mobile device, such as a cellular telephone within a cellular telephone network), several approaches use “multilateration” (“triangulation” in the case of three transmitters). Multilateration use measurements of the arrival timing of signals sent by several transmitters (such as basestations) and a received at a receiver (such as a mobile device) to determine the location of the receiver. For example, one approach, called Advanced Forward Link Trilateration (AFLT) (also known as Enhanced Observed Time Difference (EOTD)), measures at the mobile device the times of arrival of signals transmitted from each of several basestations. These arrival time measurements are transmitted to a Position Determination Entity (PDE), also called a location server, which computes the position of the mobile device using these arrival time measurements. The times-of-day at these basestations are synchronized such that the time-of-day at each basestation is the same to within a specified error. The accurate positions of the basestations and the arrival time measurements are used to determining the position of the mobile device.
FIG. 1 shows an example of an AFLT system where the arrival time measurements (TR1, TR2, and TR3) of signals from cellular basestations 101, 103, and 105 are measured at a mobile device, such as a mobile cellular telephone 111. These arrival time measurements may then be used to compute the position of the mobile device. Such computation may be done at the mobile device itself, or at a location server if the timing information obtained by the mobile device is transmitted to the location server via a communication link. Typically, the arrival time measurements are communicated to a location server 115 through one of the cellular basestations (e.g., basestation 101, or 103, or 105). The location server 115 is coupled to receive data from the basestations through the mobile switching center 113. The mobile switching center 113 provides signals (e.g., voice communications) to and from a land-line Public Switched Telephone Network (PSTN) so that signals may be conveyed to and from the mobile device to other communication devices, such as other land-line phones on the PSTN or other mobile telephones. In some cases the location server may also communicate with the mobile switching center via a cellular link. The location server may also monitor emissions from several of the basestations in an effort to determine the relative timing of these emissions.
In another method called Time Difference of Arrival (TDOA), the arrival time of a signal from a mobile device is measured at several basestations. FIG. 1 applies to this case if the arrows of TR1, TR2, and TR3 are reversed. This timing data may then be communicated to the location server to compute the position of the mobile device.
Yet a third method involves the use of a receiver in the mobile device for receiving signals from a Satellite Positioning System (SPS), such as the United States Global Positioning Satellite (GPS) system, the Russian Glonass system, the proposed European Galileo System or any other such satellite navigation system. Alternatively, a combination of satellites and “pseudolites” may be used. Pseudolites are ground based transmitters that broadcast a PN code (similar to a GPS signal) modulated on an L-band carrier signal, generally synchronized with SPS time. Each pseudolite may be assigned a unique PN code so as to permit identification by a remote receiver. Pseudolites are useful in situations where SPS signals from an orbiting satellite might be unavailable, such as in tunnels, mines, buildings or other enclosed areas. The term “navigational transmitter”, as used herein, is intended to include any satellite, communication base station, pseudolite, or equivalent of a pseudolite. The terms “airborne navigational transmitter” and terrestrial navigational transmitter” are used to distinguish between those navigational receivers that are earth bound and those that are not. The term SPS signals as used herein is intended to include any signal transmitted by a navigational transmitter. Such SPS methods may utilize a cellular network to either provide assistance data or share in the processing required to calculate the position of the SPS receiver. Alternatively, such a method may be completely autonomous (i.e., not utilize a cellular network). Examples of such a method are described in U.S. Pat. Nos. 5,841,396; 5,945,944; and 5,812,087. In practical low-cost implementations, both a cellular communications receiver of the mobile device and the SPS receiver are integrated into the same enclosure and, in some cases, may share common electronic circuitry.
A combination of either the AFLT method or TDOA method with an SPS method is referred to herein as a “hybrid” method.
In yet another variation of the above methods, the round trip delay (RTD) is calculated for signals that are sent from the basestation to the mobile device and back. In a similar, but alternative, method the round trip delay is calculated for signals that are sent from the mobile device to the basestation and back. In each of these cases, the round-trip delay is divided by two in order to determine an estimate of the one-way time delay. Knowledge of both the location of the basestation and the one-way delay constrains the location of the mobile device to a circle on the earth. If the location of a second basestation and the one-way delay from the second basestation to the mobile device is know, then the result is the intersection of two circles, which in turn constrains the location of the mobile device to two points on the earth. Knowledge of a third basestation and delay (or even an angle of arrival or information regarding the cell sector in which the mobile device resides) may resolve the ambiguity as to which of the two points is the location of the mobile device.
In a positioning system based upon multilateration, there are typically four primary unknowns. In a system based on Cartesian coordinates, the four unknowns include three components of the mobile device's position (x, y, and z, which may represent latitude, longitude, and altitude) and a “clock bias” of the mobile device. The clock bias is the difference between the time maintained by the clock in the mobile device and the time maintained in the transmitters, assuming that the time maintained in each transmitter is perfectly synchronized with each of the other transmitters. In the case of GPS satellites, the clock in each satellite is an atomic clock. The accuracy provided by the atomic clocks allows the time in each satellite to be very nearly in perfect synchronization. It will be clear to those skilled in the art that to solve for the four unknowns and form an estimate of the position of the mobile device, at least four independent equations must be obtained. If one of the four unknowns can be estimated or is known, such as the altitude, then only three independent equations are required. Knowledge of the location of a satellite, the time at which a signal was transmitted from that satellite and a measurement of the arrival time of a signal transmitted from that satellite provide sufficient information to form one independent equation. Each additional satellite and associated set of timing information adds one additional independent equation.
An iterative position determination procedure typically starts with an a-priori estimated position. Then, the estimated position is improved with each iteration. Based on the estimated position, a position correction vector and an improved estimation of the clock bias are determined. In a SPS system, four independent equations can be used to solve for four unknowns using a well known least squares iterative solution improvement approach.
The least squares solution to these equations provides an adjustment to the a-priori solution. Each measurement may also be weighted appropriately. A weighting process can help to improve accuracy when the a-priori accuracy estimate for each measurement input varies significantly. For example, U.S. Pat. No. 6,313,786 contains descriptions of an error estimation process and weighting scheme.
Altitude aiding has been used in various methods for determining the position of a mobile device. Altitude aiding is typically based on a pseudo-measurement of the altitude. Knowing the altitude of a mobile device constrains the possible positions of the mobile device to the surface of a sphere (or an ellipsoid) with the sphere's center located at the center of the earth. This knowledge may be used to reduce the number of independent arrival time measurements required to determine the position of the mobile device. Typically, an estimated altitude can be: (1) manually supplied by the operator of the mobile device, (2) set to an altitude from a previous three-dimensional solution, (3) set to a predetermined value, or (4) derived from mapping information (such as a topographical or geodetic database) maintained at a location server.
U.S. Pat. No. 6,061,018 describes a method by which an estimated altitude is determined from information of a “cell object”. The cell object is a cell site that has a cell site transmitter in communication with the mobile device. U.S. Pat. No. 6,061,018 also describes a method of determining the condition of the measurements of the pseudoranges from a plurality of SPS satellites by comparing an altitude calculated from the pseudorange measurements with the estimated altitude.